1. Field of the Invention
The present invention relates to a photovoltaic device of a high conversion efficiency, high reliability and satisfactory mass producibility, and a method for producing the same, and it also relates to a secondary battery, an artificial satellite, a roofing material, a solar cell module, an automobile or the like, utilizing a solar cell as an example of said photovoltaic device.
2. Related Background Art
For most future energy sources, there are involved various problems, e.g. in the total reliance on fossil fuels such as petroleum and coal, the use of which is considered to result in warming of the earth because of the by-product carbon dioxide etc., and on nuclear power, which cannot be freed from the danger of radioactivity in case of an unpredictable accident or utilizing solar light as an energy source have very little influence on the environment, and are expected to be used for many applications. At present, however, there are some drawbacks that hinder practical use of such solar cells,
For solar power generation, mostly monocrystalline or polycrystalline silicon have been utilized. However, such solar cells, as an example of the known photovoltaic cells, require a large amount of energy and time for crystal growth and also require complex subsequent steps, so that mass production is difficult to achieve and supply thereof at a low price has therefore been difficult. On the other hand, there have been conducted active research and development on so-called thin film semiconductor photovoltaic devices utilizing amorphous silicon (hereinafter represented as a-Si), or compound semiconductors such as CdS or CuInSe.sub.2. Such photovoltaic devices can be obtained by forming the necessary semiconductor layers on an inexpensive substrate such as glass or stainless steel with a relatively simple process, and therefore offer the possibility of cost reduction. However, such thin film semiconductor photovoltaic devices have not been employed in practice because their conversion efficiency of light is lower than that of the crystalline silicon photovoltaic devices, and also their reliability under prolonged use has been low. For this reason, there have been made various attempts to improve the performance of the thin film semiconductor photovoltaic devices.
One attempt consists of providing a rear reflective layer for returning the light not absorbed by the semiconductor layer to said semiconductor layer, in order to achieve effective utilization of the incident light. For this purpose, in the case where the light is introduced through a transparent substrate, the electrode formed on the surface of the semiconductor layer is composed of a highly reflective metal such as silver (Ag), aluminum (Al), or copper (Cu). Also, in the case where the light is introduced from the top side of the semiconductor layer, a similar metal layer for increasing the reflectance is formed on the substrate, before the formation of said semiconductor layer. Also, a transparent layer with a suitable optical property may be provided between the metal layer and the semiconductor layer, for further improving the reflectance by multiple interference. FIG. 4A shows the reflectance in the case where such transparent layer is not present between silicon and various metals, and FIG. 4B shows the simulated results of improvement in reflectance, in the case when a zinc oxide (ZnO) layer is provided as such transparent layer between silicon and various metals.
Such a transparent layer is also effective in improving the reliability of the photovoltaic device. Japanese Patent Publication No. 60-41878 discloses that such a transparent layer prevents alloy formation between the semiconductor and the metal layer. Also, U.S. Pat. Nos. 4,532,372 and 4,598,306 disclose that a transparent layer with a suitable electrical resistance prevents the generation of excessive current between the electrodes even in the case of short-circuiting in the semiconductor layer. Another attempt to improve the conversion efficiency of the photovoltaic device consists of employing a texture, having fine irregularities, on the surface of the photovoltaic device and/or the interface with the rear reflective layer. In such configuration, the light is scattered on the surface of the photovoltaic device and/or at the interface with the rear reflective layer and is confined in the semiconductor (phototrapping effect), whereby it is effectively absorbed therein. In the case where the substrate is transparent, the texture is formed on the surface of a transparent electrode, for example, of tin oxide (SnO.sub.2) formed on the substrate. Also, in the case where the light is introduced from the top side of the semiconductor, the texture is formed on the surface of the metal layer employed as the rear reflective layer. M. Harasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano, and H. Okaniwa reported that an irregular texture for a rear reflective layer could be obtained by Al deposition under control of the substrate temperature and the deposition rate (Solar Cell Materials, Vol. 20 (1990), pp. 99-110). FIG. 5 shows an example of the increase of absorption of the incident light, through the use of a rear reflective layer with such texture, wherein curve (a) indicates the spectral sensitivity of an a-Si photovoltaic device employing smooth Ag as the metal layer, while curve (b) indicates the spectral sensitivity of a similar device employing Ag of irregular texture.
It is also possible to combine the concept of the rear reflective layer consisting of a metal layer and a transparent layer, and the concept of texture structure. U.S. Pat. No. 4,419,533 discloses the concept of the rear reflective layer in which the surface of the metal layer has a textured structure and a transparent layer is formed thereon. The light conversion efficiency of the solar cell, constituting an example of the photovoltaic device, is expected to be significantly improved by such combination. However, according to the experiences of the present inventors, such improvements have not been attained in most cases. Also, under certain conditions of semiconductor deposition, the obtained solar cell does not have enough reliability for use under the conditions of high temperature and high humidity, despite the presence of the transparent layer. For these reasons, the thin film solar cells have not been employed in practical applications, though they have the possibility of low cost production.